The venv module provides support for creating lightweight “virtual
environments” with their own site directories, optionally isolated from system
site directories. Each virtual environment has its own Python binary (allowing
creation of environments with various Python versions) and can have its own
independent set of installed Python packages in its site directories.

Running this command creates the target directory (creating any parent
directories that don’t exist already) and places a pyvenv.cfg file in it
with a home key pointing to the Python installation from which the command
was run. It also creates a bin (or Scripts on Windows) subdirectory
containing a copy of the python binary (or binaries, in the case of
Windows). It also creates an (initially empty) lib/pythonX.Y/site-packages
subdirectory (on Windows, this is Lib\site-packages).

Deprecated since version 3.6: pyvenv was the recommended tool for creating virtual environments for
Python 3.3 and 3.4, and is deprecated in Python 3.6.

Changed in version 3.5: The use of venv is now recommended for creating virtual environments.

usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear]
[--upgrade] [--without-pip]
ENV_DIR [ENV_DIR ...]
Creates virtual Python environments in one or more target directories.
positional arguments:
ENV_DIR A directory to create the environment in.
optional arguments:
-h, --help show this help message and exit
--system-site-packages
Give the virtual environment access to the system
site-packages dir.
--symlinks Try to use symlinks rather than copies, when symlinks
are not the default for the platform.
--copies Try to use copies rather than symlinks, even when
symlinks are the default for the platform.
--clear Delete the contents of the environment directory if it
already exists, before environment creation.
--upgrade Upgrade the environment directory to use this version
of Python, assuming Python has been upgraded in-place.
--without-pip Skips installing or upgrading pip in the virtual
environment (pip is bootstrapped by default)
Once an environment has been created, you may wish to activate it, e.g. by
sourcing an activate script in its bin directory.

Changed in version 3.4: Installs pip by default, added the --without-pip and --copies
options

Changed in version 3.4: In earlier versions, if the target directory already existed, an error was
raised, unless the --clear or --upgrade option was provided. Now,
if an existing directory is specified, its contents are removed and
the directory is processed as if it had been newly created.

The created pyvenv.cfg file also includes the
include-system-site-packages key, set to true if venv is
run with the --system-site-packages option, false otherwise.

Unless the --without-pip option is given, ensurepip will be
invoked to bootstrap pip into the virtual environment.

Multiple paths can be given to venv, in which case an identical virtual
environment will be created, according to the given options, at each provided
path.

Once a virtual environment has been created, it can be “activated” using a
script in the virtual environment’s binary directory. The invocation of the
script is platform-specific:

Platform

Shell

Command to activate virtual environment

Posix

bash/zsh

$ source <venv>/bin/activate

fish

$ . <venv>/bin/activate.fish

csh/tcsh

$ source <venv>/bin/activate.csh

Windows

cmd.exe

C:\> <venv>\Scripts\activate.bat

PowerShell

PS C:\> <venv>\Scripts\Activate.ps1

You don’t specifically need to activate an environment; activation just
prepends the virtual environment’s binary directory to your path, so that
“python” invokes the virtual environment’s Python interpreter and you can run
installed scripts without having to use their full path. However, all scripts
installed in a virtual environment should be runnable without activating it,
and run with the virtual environment’s Python automatically.

You can deactivate a virtual environment by typing “deactivate” in your shell.
The exact mechanism is platform-specific: for example, the Bash activation
script defines a “deactivate” function, whereas on Windows there are separate
scripts called deactivate.bat and Deactivate.ps1 which are installed
when the virtual environment is created.

New in version 3.4: fish and csh activation scripts.

Note

A virtual environment is a Python environment such that the Python
interpreter, libraries and scripts installed into it are isolated from those
installed in other virtual environments, and (by default) any libraries
installed in a “system” Python, i.e., one which is installed as part of your
operating system.

A virtual environment is a directory tree which contains Python executable
files and other files which indicate that it is a virtual environment.

Common installation tools such as Setuptools and pip work as
expected with virtual environments. In other words, when a virtual
environment is active, they install Python packages into the virtual
environment without needing to be told to do so explicitly.

When a virtual environment is active, any options that change the
installation path will be ignored from all distutils configuration files to
prevent projects being inadvertently installed outside of the virtual
environment.

When working in a command shell, users can make a virtual environment active
by running an activate script in the virtual environment’s executables
directory (the precise filename is shell-dependent), which prepends the
virtual environment’s directory for executables to the PATH environment
variable for the running shell. There should be no need in other
circumstances to activate a virtual environment—scripts installed into
virtual environments have a “shebang” line which points to the virtual
environment’s Python interpreter. This means that the script will run with
that interpreter regardless of the value of PATH. On Windows, “shebang”
line processing is supported if you have the Python Launcher for Windows
installed (this was added to Python in 3.3 - see PEP 397 for more
details). Thus, double-clicking an installed script in a Windows Explorer
window should run the script with the correct interpreter without there
needing to be any reference to its virtual environment in PATH.

The high-level method described above makes use of a simple API which provides
mechanisms for third-party virtual environment creators to customize environment
creation according to their needs, the EnvBuilder class.

The EnvBuilder class accepts the following keyword arguments on
instantiation:

system_site_packages – a Boolean value indicating that the system Python
site-packages should be available to the environment (defaults to False).

clear – a Boolean value which, if true, will delete the contents of
any existing target directory, before creating the environment.

symlinks – a Boolean value indicating whether to attempt to symlink the
Python binary (and any necessary DLLs or other binaries,
e.g. pythonw.exe), rather than copying. Defaults to True on Linux and
Unix systems, but False on Windows.

upgrade – a Boolean value which, if true, will upgrade an existing
environment with the running Python - for use when that Python has been
upgraded in-place (defaults to False).

with_pip – a Boolean value which, if true, ensures pip is
installed in the virtual environment. This uses ensurepip with
the --default-pip option.

prompt – a String to be used after virtual environment is activated
(defaults to None which means directory name of the environment would
be used).

Changed in version 3.4: Added the with_pip parameter

New in version 3.6: Added the prompt parameter

Creators of third-party virtual environment tools will be free to use the
provided EnvBuilder class as a base class.

This method takes as required argument the path (absolute or relative to
the current directory) of the target directory which is to contain the
virtual environment. The create method will either create the
environment in the specified directory, or raise an appropriate
exception.

The create method of the EnvBuilder class illustrates the hooks
available for subclass customization:

defcreate(self,env_dir):""" Create a virtualized Python environment in a directory. env_dir is the target directory to create an environment in. """env_dir=os.path.abspath(env_dir)context=self.ensure_directories(env_dir)self.create_configuration(context)self.setup_python(context)self.setup_scripts(context)self.post_setup(context)

Creates the environment directory and all necessary directories, and
returns a context object. This is just a holder for attributes (such as
paths), for use by the other methods. The directories are allowed to
exist already, as long as either clear or upgrade were
specified to allow operating on an existing environment directory.

Creates a copy of the Python executable (and, under Windows, DLLs) in
the environment. On a POSIX system, if a specific executable
python3.x was used, symlinks to python and python3 will be
created pointing to that executable, unless files with those names
already exist.

path is the path to a directory that should contain subdirectories
“common”, “posix”, “nt”, each containing scripts destined for the bin
directory in the environment. The contents of “common” and the
directory corresponding to os.name are copied after some text
replacement of placeholders:

__VENV_DIR__ is replaced with the absolute path of the environment
directory.

__VENV_NAME__ is replaced with the environment name (final path
segment of environment directory).

__VENV_PROMPT__ is replaced with the prompt (the environment
name surrounded by parentheses and with a following space)

__VENV_BIN_NAME__ is replaced with the name of the bin directory
(either bin or Scripts).

__VENV_PYTHON__ is replaced with the absolute path of the
environment’s executable.

The directories are allowed to exist (for when an existing environment
is being upgraded).

The following script shows how to extend EnvBuilder by implementing a
subclass which installs setuptools and pip into a created virtual environment:

importosimportos.pathfromsubprocessimportPopen,PIPEimportsysfromthreadingimportThreadfromurllib.parseimporturlparsefromurllib.requestimporturlretrieveimportvenvclassExtendedEnvBuilder(venv.EnvBuilder):""" This builder installs setuptools and pip so that you can pip or easy_install other packages into the created virtual environment. :param nodist: If True, setuptools and pip are not installed into the created virtual environment. :param nopip: If True, pip is not installed into the created virtual environment. :param progress: If setuptools or pip are installed, the progress of the installation can be monitored by passing a progress callable. If specified, it is called with two arguments: a string indicating some progress, and a context indicating where the string is coming from. The context argument can have one of three values: 'main', indicating that it is called from virtualize() itself, and 'stdout' and 'stderr', which are obtained by reading lines from the output streams of a subprocess which is used to install the app. If a callable is not specified, default progress information is output to sys.stderr. """def__init__(self,*args,**kwargs):self.nodist=kwargs.pop('nodist',False)self.nopip=kwargs.pop('nopip',False)self.progress=kwargs.pop('progress',None)self.verbose=kwargs.pop('verbose',False)super().__init__(*args,**kwargs)defpost_setup(self,context):""" Set up any packages which need to be pre-installed into the virtual environment being created. :param context: The information for the virtual environment creation request being processed. """os.environ['VIRTUAL_ENV']=context.env_dirifnotself.nodist:self.install_setuptools(context)# Can't install pip without setuptoolsifnotself.nopipandnotself.nodist:self.install_pip(context)defreader(self,stream,context):""" Read lines from a subprocess' output stream and either pass to a progress callable (if specified) or write progress information to sys.stderr. """progress=self.progresswhileTrue:s=stream.readline()ifnots:breakifprogressisnotNone:progress(s,context)else:ifnotself.verbose:sys.stderr.write('.')else:sys.stderr.write(s.decode('utf-8'))sys.stderr.flush()stream.close()definstall_script(self,context,name,url):_,_,path,_,_,_=urlparse(url)fn=os.path.split(path)[-1]binpath=context.bin_pathdistpath=os.path.join(binpath,fn)# Download script into the virtual environment's binaries folderurlretrieve(url,distpath)progress=self.progressifself.verbose:term='\n'else:term=''ifprogressisnotNone:progress('Installing %s ...%s'%(name,term),'main')else:sys.stderr.write('Installing %s ...%s'%(name,term))sys.stderr.flush()# Install in the virtual environmentargs=[context.env_exe,fn]p=Popen(args,stdout=PIPE,stderr=PIPE,cwd=binpath)t1=Thread(target=self.reader,args=(p.stdout,'stdout'))t1.start()t2=Thread(target=self.reader,args=(p.stderr,'stderr'))t2.start()p.wait()t1.join()t2.join()ifprogressisnotNone:progress('done.','main')else:sys.stderr.write('done.\n')# Clean up - no longer neededos.unlink(distpath)definstall_setuptools(self,context):""" Install setuptools in the virtual environment. :param context: The information for the virtual environment creation request being processed. """url='https://bitbucket.org/pypa/setuptools/downloads/ez_setup.py'self.install_script(context,'setuptools',url)# clear up the setuptools archive which gets downloadedpred=lambdao:o.startswith('setuptools-')ando.endswith('.tar.gz')files=filter(pred,os.listdir(context.bin_path))forfinfiles:f=os.path.join(context.bin_path,f)os.unlink(f)definstall_pip(self,context):""" Install pip in the virtual environment. :param context: The information for the virtual environment creation request being processed. """url='https://raw.github.com/pypa/pip/master/contrib/get-pip.py'self.install_script(context,'pip',url)defmain(args=None):compatible=Trueifsys.version_info<(3,3):compatible=Falseelifnothasattr(sys,'base_prefix'):compatible=Falseifnotcompatible:raiseValueError('This script is only for use with ''Python 3.3 or later')else:importargparseparser=argparse.ArgumentParser(prog=__name__,description='Creates virtual Python ''environments in one or ''more target ''directories.')parser.add_argument('dirs',metavar='ENV_DIR',nargs='+',help='A directory in which to create the'virtual environment.')parser.add_argument('--no-setuptools',default=False,action='store_true',dest='nodist',help="Don't install setuptools or pip in the ""virtual environment.")parser.add_argument('--no-pip',default=False,action='store_true',dest='nopip',help="Don't install pip in the virtual ""environment.")parser.add_argument('--system-site-packages',default=False,action='store_true',dest='system_site',help='Give the virtual environment access to the ''system site-packages dir.')ifos.name=='nt':use_symlinks=Falseelse:use_symlinks=Trueparser.add_argument('--symlinks',default=use_symlinks,action='store_true',dest='symlinks',help='Try to use symlinks rather than copies, ''when symlinks are not the default for ''the platform.')parser.add_argument('--clear',default=False,action='store_true',dest='clear',help='Delete the contents of the ''virtual environment ''directory if it already ''exists, before virtual ''environment creation.')parser.add_argument('--upgrade',default=False,action='store_true',dest='upgrade',help='Upgrade the virtual ''environment directory to ''use this version of ''Python, assuming Python ''has been upgraded ''in-place.')parser.add_argument('--verbose',default=False,action='store_true',dest='verbose',help='Display the output ''from the scripts which ''install setuptools and pip.')options=parser.parse_args(args)ifoptions.upgradeandoptions.clear:raiseValueError('you cannot supply --upgrade and --clear together.')builder=ExtendedEnvBuilder(system_site_packages=options.system_site,clear=options.clear,symlinks=options.symlinks,upgrade=options.upgrade,nodist=options.nodist,nopip=options.nopip,verbose=options.verbose)fordinoptions.dirs:builder.create(d)if__name__=='__main__':rc=1try:main()rc=0exceptExceptionase:print('Error: %s'%e,file=sys.stderr)sys.exit(rc)